My 2000 xj8 sat for a week, The battery went down due to a small drain somewhere in the elec system. This was not unexpected. The major problem occurred when i started by accidentally crossing the jumper cables on the donor vehicle. I attempted to start the Jag...Nothing. Returning to the trunk, i noticed the cables smoking. I corrected polarity. The engine started and ran roughly for about 1/2 minute and cut off.
I restarted the motor which cut off after 10 seconds. Motor cranks but unable to start it again. Have checked the 5 fuse banks. One 5Amp fuse in the trunk was blown which i replaced. any thoughts on what could have been damaged to prevent it from starting ?

 

Check the fuses again, and post which fuse was originally blown, and what is blown now. I would not expect the fuel pump circuit to be highly sensitive to crossed polarity , but the fact that it ran for 10 seconds might be a clue. Other than than that, check for spark and injector pulses.